1. Field of the Invention
The present invention relates to an ion current generator employed in thin film formation process, ion implantation process, etching process, sputtering process and the like.
2. Description of the Prior Art
FIG. 1 is a cross section of an ion beam epitaxial growth system having a conventional ion current generator which is disclosed in Japanese Patent Laying Open Gazette No. 60-137012 (1985), for example. As shown in FIG. 1, the system comprises an ion source 1 and an extraction electrode 2 provided for extracting ions from the ion source 1. A mass analyzing system 3 of a three-dimensional focusing sector type is also provided for extracting ions of desired species from the ion beam obtained through the extraction electrode 2. A resolved aperture 4 is positioned on the exit side of the mass analyzing system 3. A deceleration system 5 which consists of three cylindrical lens 5A, 5B and 5C aligned in series is also provided for decelerating the ion beam having passed through the resolving aperature 4. In a vaporizer 6 provided at the position under the exit side of the deceleration system 5, atomic materials prepared for thin film formation is vaporized, to generate a vapor current. A substrate 7 on which a thin film is to be formed by means of the system is positioned on the exit side of the deceleration system 5. The voltage distribution indicated at the ion source 1, the extraction electrode 2, the mass analyzing system 3 and the deceleration system 5 respectively are examples suitable for supplying an As.sup.+ beam of 100 eV to the substrate 7 from the ion source 1.
In process of growing a GaAs compound semiconductor thin film on the substrate 7, for example, Ga vapor is supplied to the surface of the substrate 7 from the vaporizer 6 for depositing Ga atoms on the substrate 7. Simultaneously, an extraction voltage of about 25 KV is applied between the extraction electrode 2 and the ion source 1, so that an ion beam having As.sup.+ ions is extracted from the ion source 1. The extracted ion beam is introduced into the mass analyzing system 3 of the three-dimensional sector type, through which only a pure As.sup.+ beam is extracted from the ion beam having various ions. Then, the As.sup.+ beam enters the resolving aperture 4. the As.sup.+ beam having passed through the resolving aperture 4 is decelerated through the deceleration system 5. The As.sup.+ beam is implanted into the substrate 7 after being decelerated to a low energy state of about 100 eV or lower than the same. As a result, the GaAs thin film is formed on the substrate 7.
When the conventional ion beam epitaxial grown system having the ion current generator is employed, a considerably high voltage of about 25 KV should be applied between the ion source 1 and the extraction electrode 2 so that the As.sup.+ beam having a desired electric current may be obtained. The ion beam extracted through the extraction voltage of about 25 KV has high current velocity. In order to prevent the phenomena in which the beam spreads by a space charge effect in the beam pass from the mass analyzing system 3 to the substrate 7, the electric potential of the ion source 1 is held at 100V and the mass analyzing system 3 and the deceleration system 5 are held at deep negative potentials to maintain the high beam velocity. On the contrary, the beam velocity should be low when the beam is supplied to the substrate 7. More particularly, the beam velocity must be lowered so that the incident energy of the ion beam into the substrate 7 is lower than 300 eV, or preferably lower than 100 eV. This is because, if the As.sup.+ beam is supplied to the substrate 7 with an incident velocity corresponding to an energy in excess of 300 eV, the amount of GaAs sputtered by As.sup.+ ions is equal to or in excess of the amount of GaAs adhered to the substrate 7, so that the film is prevented from growing. Therefore, the ion beam should be decelerated just before reaching the substrate 7, and the deceleration is achieved by means of the deceleration system 5. However, in the conventional thin film formation system, an electrode provided in the deceleration system 5 should be long in the beam pass direction, since the beam must be decelerated at wide range. As a result, the raster scan of the ion beam supplied to the substrate 7 cannot be well controlled. Even if the raster scan is controlled, it is difficult to obtain an uniform distribution of the fim thickness, and it is almost impossible to selectively grow a thin film only on a local area on the substrate 7.
Furthermore, since the ion beam having desired purity is obtained through the process in which the original ion beam having various ions is extracted from the ion source 1 and then given to the mass analyzing system 3, the generation efficiency of the ion beam is low, and the cost and the size of the ion generation part are increased.
FIG. 2 is a cross section showing another conventional ion current generator employing a laser, which is disclosed Japanese Patent Laying Open Gazette 50-22999 (1975). As shown in FIG. 2, the ion current generator comprises a particle current generator 60 for supplying the materials to be ionized as a form of atom current 65 moving in a certain direction, and dye laser oscillators 61a, 61b and 61c for radiating lasers having uniform wavelengths being different from each other, respectively.
Lasers 62a, 62b and 62c are provided for focusing the lasers 67a, 67b and 67c radiated from the dye laser oscillators 61a, 61b and 61c at a common point P, respectively. With an electrode 64, only ions are extracted from an atomic current 63 including the ions obtained by ionizing a part of the atomic current 65 by the lasers 67a, 67b and 67c, to be led to a certain direction.
When the ion current generator is employed for ionizing Na atoms, the Na atoms are supplied to the particle current generator 60, and the atom current 65 of Na is emitted from a nozzle 66 to a certain direction with a constant velocity. At the point P, the atom current 65 is irradiated with the laser 67a (589 nm) and the laser 67b (568.8 nm). As a result, the Na atoms are excited from a ground state 3s.sup.2 S.sub.1/2 to a 4d state a through 3p.sup.2 P.sub.3/2 state.
FIG. 3 is an energy level diagram of a Na atom. As understood from FIG. 3, the 4d state of the Na atom exists under the ionization limit level by 7000 cm.sup.-1. When the third laser oscillator 61c is adjusted so that the laser 67c may have wavelength shorter than 1.4 .mu.m, the Na atom in the 4d state is directly ionized by the laser 67c. Accordingly, the atom current 63 having passed the point P includes ions partially, and it is supplied to the electrode 64. Since a uniform electric field is being generated by the electrode 64, only the ions included in the atom current 63 are deflected by the electric field. As a result, only the ion are extracted to a certain direction.
Power densities of the lasers 67a, 67b and 67c required for ionizing the Na atoms with a high efficiency at the point P are about 10 W/cm.sup.2, about 40 W/cm.sup.2 and about 10.sup.7 W/cm.sup.2, respectively, provided that the line width of the lasers are identical with the absorption wavelength band width of respective transitions. Namely, since Einstein's A coefficient in the transition of the Na atom from the 3s.sup.2 S.sub.1/2 state to the 3p.sup.2 P.sub.3/2 state (transition wavelength of 589 nm is about 6.3.times.10.sup.7 sec.sup.-1, the minimum power density of the laser 67a required for saturating the excitation from the groud state to the 3p.sup.2 P.sub.3/2 state is about 10 W/cm.sup.2. Since Einstein's A coefficient in the transition of the Na atom from the 3p.sup.2 P.sub.3/2 state to the 4d state (transition wavelength of 568.8 nm) is about 1.3.times.10.sup.7 sec.sup.-1, the minimum power density of laser 67b required for saturating the excitation from the 3p.sup.2 P.sub.3/2 state to the 4d state is about 40 W/cm.sup.2. Furthermore, since the absorption cross section of a light corresponding to the ionization of the 4d Na atom is about 10.sup.-18 cm.sup.2, the minimum power density of the laser 67c required for ionizing the 4d state Na atom is about 10.sup.7 W/cm.sup.2.
Accordingly, a laser having power density larger than 10.sup.7 W/cm.sup.2 should be applied to the Na atom to ionize the Na atom by the laser.
In order to increase the power density of the laser, the laser may be focused. However, when such a technique is employed, atoms only in a small region are ionized, so that the amount of the obtained ions is decreased.
Further, the laser cannot be focused on an area having a diameter smaller than about several ten .mu.m according to the present level of a laser technology, and therefore, the area is about several 10.sup.-5 cm.sup.2 in maximum. On the other hand, the output energy of a continuous laser oscillator is smaller than 1 W, so that the maximum power density obtained by the laser oscillator is the order of 10.sup.5 W/cm.sup.2. Thus, the continuous laser oscillator cannot be employed as the third laser oscillator 61c.
On the contrary, the maximum output of about 10.sup.6 W can be obtained in a pulse dye laser oscillator available commercially. When the pulse dye laser oscillator is employed, a desired ion beam can be generated. However, in the pulse laser oscillator, the amount of the ion obtained per unit time is proportial to the frequency of the pulse oscillation, provided that the laser radiation time per one pulse is constant. For this reason, a pulse dye laser oscillator with the high oscillation frequency should be employed when a large amount of the ions are required.
When the ion density of the ion beam is more than 10.sup.10 cm.sup.-3, the special field generated by the ions themselves exceeds 3 KV/cm, and the ions undesirably spread in the path from the point P to the electrode 64. Therefore, the maximum ion density of the ions entering the electrode 64 is 10.sup.10 cm.sup.-3.
In general, the value of the current density j (A/cm.sup.2) is calculated through the following formula: EQU j=N.sub.i ef.sub.L ( 1)
where,
n.sub.i : the amount of the ions per one pulse, PA1 e: the charge of an electron, and PA1 f.sub.L : the oscillation frequency of a laser.
The oscillation frequency of a pulse dye laser oscillator is about 1 KHz in maximum. Assuming that the amount n.sub.i of the ions per one pulse, the charge e of the electron and the repeatation frequency f.sub.L of the laser are 10.sup.10 cm.sup.-3, 1.6.times.10.sup.-19 Coulomb and 1 KHz, respectively, these values are substituted in the formula (1) to give the current density j as 1.6.times.10.sup.-6 (A/cm.sup.2). Thus, in a case that the pulse dye laser oscillator is employed, the current density of the ion beam outputted therefrom is about several one .mu. A/cm.sup.2, and an ion beam having a large current density is hardly obtained.
Furthermore, since the life time of the pulse dye laser oscillator is over when it has oscillated by 10.sup.9 shorts, the life time is estimated as about 300 hours in a case that the oscillation frequency of the oscillator is 1 KHz, so that the laser oscillation often stops undesirably and the oscillator should be often repaired.